By comparing the amounts of parent and daughter

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By comparing the amounts of parent and daughter isotopes in the rock, we can determine the fraction of parent isotopes that has decayed since the rock formed Conclusion: A radiometric age will be reliable if we can be confident in knowing precisely how much of the original parent isotope has decayed, and any uncertainty in decay fraction will mean a corresponding uncertainty in the age. What does the geological record show? Most fossils contain little or no organic matter In some cases, the mineral replacement is complete and organisms literally turn to stone In many other cases, the organisms themselves decay, but in doing so they leave an empty mold that fills with minerals dissolved in water Coprolites are rocks that consist of petrified excrement, which can allow us to learn about an animal’s diet First 90% of Earth’s history has microscopic organisms Geological time scale Four eons: Hadean, Archean, Proterozoic, Phanerozoic Phanerozoic – Plaeozoic, Mesozoic, and Cenozoic Studies of tiny mineral grains of zirconium silicate Lunar rocks While the zircons and lunar rocks set a min age for Earth, we can set a max age by dating the formation of the solar system as a whole by studying meteorites Earth and Moon formed within about 50 to 70 million years after the oldest meteorites formed Conclusion: By about 4.5 billion years ago, our planet had its Moon and must have been essentially at its current mass and size, ready for its geology to begin shaping the features that would ultimately make it our home. How did Earth get atmosphere and oceans? Planetesimals were flung inward by gravitational interactions with other planetesimals and forming planets Some of these planetesimals came from far enough away and contained ices or rock chemically bound with molecules of water or other common gases When molten rock erupts onto the surface as lava, the release of pressure violently expels the trapped gas in a process we call outgassing Some water and gas may have been supplied directly to the surface by impacts after Earth formed. This process may have begun to create an atmosphere even before Earth was fully formed Volcanism is the major source of outgassing Water vapor condensed as rain to fill Earth’s oceans and gases that remained airborne made up Earth’s early atmosphere Earth may have had early continents, oceans, and an atmosphere within only about 100 million years after the planet first formed Early atmosphere was dominated by CO2, while today’s atmosphere is dominated by N and some O Conclusion: Earth’s present oxygen atmosphere is almost certainly a result of photosynthesis by living organisms.
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